Recent years have seen the rapid development of photo-sensing charge coupled devices (CCD's) for electronic imaging of a scene. Because of their many advantages (small size, low power, low cost, etc.), CCD's have become the imaging units of choice in many applications such as consumer camcorders. As the performance and quality of CCD's improves, they are being used more and more in various imaging systems requiring high resolution, full color balance, wide dynamic range, low-light sensitivity, and high frequency response such as required by high definition television (HDTV) or as needed in instruments used for astronomical observations where incident light levels are extremely low.
A CCD photo-imaging unit typically has horizontal rows and vertical columns arranged in an X and Y pattern of light-sensing cells within a given area onto which an image of a scene is optically focused. By way of example, there may be about a thousand or more such cells in each vertical column and a thousand or more cells in each horizontal row for a total of a million or more cells within an area which may be only one or a few square centimeters. Each cell represents a very small area, termed a pixel, of the total image; the more pixels present in the CCD unit, the higher the resolution in the image reproduced by the CCD. For prospective HDTV applications, about two million pixels per image area of a CCD are used, and the pixel signals are sampled and outputted from the unit as analog image signal voltages to a signal processor at about a 40 MHz rate. This is a much higher rate than is required, for example, in a present day color television system operating in accordance with the national television standards committee (NTSC) requirements and having only about one-quarter million pixels. This high speed of 40 MHz is difficult to achieve along with very low noise, good color balance, and linearity over a wide dynamic range.
There are certain characteristics of a CCD photo-imaging unit which must be compensated for by the electronic analog signal circuitry which receives and processes the video signals produced by the CCD unit in order to obtain a high quality image. The electrical signal stored at each cell of the CCD unit is related to the intensity of the light of an image at a given instant incident on the area of that particular cell. The individual cells are made very small (e.g., about 7 microns square) in order to obtain a large number of pixels per area (e.g., two million per area). As a consequence of the small size of each cell the electrical signal from each cell (representing an image pixel) is relatively small. Therefore noise, such as thermal noise and switching transients within the CCD unit, becomes a significant factor affecting the quality of an image reproduced by the unit.
An extensive discussion of CCD imaging units and some of the problems associated with them is given in an article by M. H. White, et al., entitled "Characterization of Surface Channel CCD Image Arrays at Low Light Levels", IEEE Journal of Solid State Circuits, vol. SC-9, No. 1, February 1974, pages 1-14. This article describes the theory and operation of a CCD imaging unit and describes a method termed correlated double sampling (CDS) "to remove switching transients, eliminate the Nyquist noise associated with the reset switch/node capacitance combination, and suppress `1/f` surface-state noise contributions of a CCD unit". A schematic diagram of a CDS signal processor employing the method of correlated double sampling is shown in FIG. 5, on page 4 of the White et al. article.
The dynamic range of an analog image signal is conveniently expressed as a binary bit number. Thus an 8-bit number (with a decimal equivalent of 256) expresses the ability of a circuit to divide (digitize) the signal accurately into 256 parts. This in turn implies that noise and distortion contribute less than one part in 256 parts (about 1/2 percent) of the total image signal. By way of example, consumer camcorders have a dynamic range of about 8-bits, whereas quality color film today has a dynamic range of roughly 12 to 14-bits (with a decimal equivalent of 4096 to 16,384). it is desirable therefore, in producing high quality electronic color images, such as desired for HDTV systems, to provide an analog signal processor (ASP) for the output of a CCD imaging unit having a dynamic range substantially greater than 8-bits.
The individual cells of a CCD unit are adapted by means of respective color masks (filters) applied over the cells to respond to individual color components of an image. In accordance with the NTSC standards for color television, the color components of a full color image are defined to be "red", "green" and "blue". In the past these red (R), green (G) and blue (B) color components were obtained directly from a CCD unit by covering certain cells of the unit with R masks, other cells with G masks, and still other cells with B masks.
By conventional definition, the color "cyan" (C) is "green" plus "blue" (G+B) and the color "yellow" (Y) is "green" plus "red" (G+R). It is known that for blue light a cyan color (C) mask (filter) has a substantially greater light transmittance than a blue (B) mask, and similarly a yellow color (Y) mask has a greater light transmittance for red light than a red (R) mask. Therefore, from the standpoint of higher electrical efficiency and lower noise, it is desirable that the cells of a CCD color imaging unit be covered with C, Y and G masks, rather than with R, G and B masks. With previous systems the difficulty and added complexity of converting C, Y, G image signals into R, G, B image signals has limited the use of CCD units which output C, Y and G color-component pixel image signals, even though such CCD units are superior in certain ways.
The "green" masked cells of a CCD color imaging unit generate (for a given "white" or balanced color image) different electrical output signals than do the "cyan" or the "yellow" cells. It is necessary therefore to compensate for these differences in the C, Y and G signal outputs in order to obtain a proper "white balance" in an electronically reproduced image. When the respective cells (e.g., C, Y and G) do not receive any light (total darkness), they in fact produce a small minimum "dark" signal voltage. The cells themselves are all the same (only the color masks are different) and the "dark" (no light) signals are substantially the same for all of the cells in a given horizontal row of a CCD unit. As will be explained in detail hereinafter, unless this "dark" signal characteristic of the C, Y and G cells is properly compensated for (along with white balancing of the respective colors) there will appear in a high resolution color image, such as in a HDTV system, visually objectionable "streaks" or variations in the "dark" background of the color image. The present invention also provides a highly effective answer to this problem of proper white balance and of reduction in dark background variations.
It is desirable to have an electronic imaging system, and an analog signal processor therefor, which are versatile in application and which provide:
1. System output of color-component image signals with one sequence of colors (i.e., color component group) derived from a CCD imaging unit which outputs color-component pixel image signals with a different sequence of colors for improved efficiency.
2. Operation over a wide range of signal sampling rates (e.g., 1 to 40 MHz).
3. Effective "white balance" of the component colors of a full color image along with minimal dark background variation.
4. Periodic "dark" pixel referencing (line rate clamping).
5. Dynamic range of 10-bits or better at slow sample rates and better than 8-bits at 40 MHz.
6. Improved clamp, sample and signal hold operation (modified "correlated double sampling") for more efficient utilization and higher signal to noise ratio (S/N) of the output signals of the CCD imaging unit.
7. Extremely stable and linear operation with efficient temperature compensation.
8. Ease of implementation as an integrated circuit.
9. Very small physical size.
10. Cost effectiveness.